In certain types of surgery it becomes necessary to drill an annular hole into a bone. For example, certain compression fractures can be treated by drilling an annulus into the bone in the area of the fracture and pressing the fractured section of bone into its original position. The position of the bone is then secured with a screw. This procedure is particularly useful in treating compression fractures in the bones of the leg and knee. In another application, drilling an annulus into the bone is necessary in order to harvest a plug of healthy bone for grafting into an area of bone which is damaged or diseased.
A trephine instrument which is commonly used to perform such surgery comprises an annular drill with a pointed projection in the center of the annulus. The projection typically extends 5-10 millimeters beyond the cutting face, and is used to guide the path of the drill. It does not always do so satisfactorily, however, and the drill may deviate from its intended path. The projection can also injure soft tissue beyond the bone, such as brain tissue, arteries, or nerves. In addition, it is often difficult to remove the drill without removing all or part of the bone plug with it, since the plug tends to bind to the instrument.
Another type of trephine instrument currently in use has an annular cutting face, but does not employ a projection in the center of the cutting face. This instrument is similar to annular drills used in carpentry. The problem of deviation from the desired path is exacerbated in this type of instrument, since the drill is not guided by anything except the cutting face itself. Cleanly removing the instrument from the bone also continues to be a problem.